1. Field of the Invention
This invention relates to a binary circuit applied to binarization of an input signal, and more particularly to binary circuit adapted to output image pick-up signals obtained (sampled) from positions spatially separated of an object to be imaged as a binary signal, and a CCD solid-state image pick-up apparatus provided with such a binary circuit. Especially, the CCD image pick-up apparatus provided with such binary circuit is suitable when used as a bar code reader adapted for reading a mark such as a bar code, etc. attached on a medium to convert it into a binary signal.
2. Description of the Related Arts
Generally, bar codes are adapted to indicate information such as name of manufacturer (marker) or name of goods, etc. on a medium, etc. by combination of lines of different thicknesses, and such bar codes are read by an optical detecting method and are utilized for aggregate (sum total) of the amount sold of goods or analysis of circulation thereof, etc.
As an apparatus for reading bar codes by the optical detecting method, a CCD image sensor is mainly used. Namely, an output from the CCD image sensor is delivered to a binary circuit to take out combination of lines of different thicknesses as binary information to detect the binary information thus taken out as bar code information. In this case, there is generally employed a method of obtaining binary information while comparing a level (voltage) of an image pick-up signal with a threshold voltage at a comparator.
However, in binarization of an output from the CCD image sensor, since reflection of a print surface of the bar code does not become uniform by the influence of unevenness of the surface of a medium on which bar codes are, printed, or an extraneous light, it is difficult to allow output data to be binary information (i.e., binarize output data ) while maintaining the threshold voltage at a fixed value. For this reason, in the prior art, a circuit adapted for inverting a current image pick-up signal when a change of the signal level more than a predetermined absolute value takes place relative to an image pick-up signal immediately before is made up outside the CCD image sensor.
A conventional bar code reader, will now be described with reference to FIG. 1. A CCD image sensor constituting this bar code reader includes an image pick-up region 2 where a large number of light receiving portion 1 for converting an incident light into a signal charge quantity corresponding to a light quantity thereof are arranged, and a charge transfer section 3 for unidirectionally transferring signal charges from the image pick-up region 2. At the succeeding stage of a charge voltage converting section 4 comprised of, e.g., floating gate formed in the vicinity of the final stage in the charge transfer section 3 of this CCD image sensor, an amplifier 5 comprised of, e.g., a source-follower circuit for current-amplifying an image pick-up signal Va from the voltage converting section 4 is formed on the same substrate.
To external terminal .phi. drawn from the amplifier 5 to the external, an amplifier 6 for level-amplifying the image pick-up signal Va current-amplified by the amplifier 5, and a binary circuit 10 for inverting a current image pick-up signal when a change of the signal level more than a predetermined absolute value takes place relative to an image pick-up signal immediately before are connected. Thus, bar code reader is constituted. In this bar code reader, as shown in the figure, external amplifier 6 having high degree of amplification is selected, and a logic circuit using diodes D7 and D8 is used as binary circuit 10 connected to the external. The input/output characteristic of the binary circuit 10 has as shown in FIG. 2, a hysteresis characteristic such that in the case where an output voltage Vout is, e.g., 5 volts which is the power supply voltage when input voltage Vin is, e.g., zero volts, output voltage Vout is inverted to zero volts when input voltage Vin is increased from zero volts by about 0.8 volts which is a voltage drop level of diodes D7 and D8, while in the case where output voltage Vout is zero volts when input voltage Vin is, e.g., 5 volts, output voltage Vout is inverted to 5 volts when input voltage Vin is decreased from 5 volts by about 0.8 volts which is the voltage drop level of diodes D7 and D8.
Accordingly, when the signal level of a current image pick-up signal Va varies by more than .+-.0.8 volts relative to the signal level of the last image pick-up signal Va, the output is inverted. Thus, it is possible to take out this inverted output as binary information.
However, since the binary circuit 10 used in the conventional bar code reader requires a logic circuit using amplifier 8 of high degree of amplification and diodes D7 and D8, the circuit configuration itself becomes complicated. Accordingly, it is difficult to make up such binary circuit within the CCD image sensor, i.e., to allow it to have on-chip configuration. Particularly, it is still more difficult to embody such binary circuit as on-chip configuration within a CCD image sensor driven by a low voltage such as power supply voltage of 3 volts, etc.
The reason whey such difficulty exists is that in the case where a logic circuit using diodes D7 and D8 is used, about 0.8 volts corresponding to voltage drop level V.sub.BE of diodes D7 an D8 becomes a dead zone in the hysteresis characteristic. Accordingly, a voltage larger than 2.about.3 volts which is more than 0.8 volts is required as the signal amplitude within the CCD image sensor. It is extremely difficult to deal with such a large signal amplitude in the CCD image sensor having power supply of 3 volts.
Further, for the purpose of obtaining in practice binary information with high accuracy by this binary circuit 10, the circuit configuration shown in FIG. 1 is insufficient. For this reason, there are also instances where an offset circuit is inserted between amplifier 6 and binary circuit 10 to deliver an output from the binary circuit 10 to a microcomputer connected to the external to allow the offset circuit to undergo feedback control by this microcomputer to further control the gain of amplifier 6.
As stated above, the circuit configuration of binary circuit 10 used in the conventional bar code reader is very complicated, and a greater number of parts are required. As a result, there was the problem that binary circuit 10 cannot be embodied as on-chip configuration within the CCD image sensor, so there takes place limitation in miniaturization of the size of the bar code reader itself. In addition, there is the problem that power consumption is increased, so this does not suit with current tendency of low level power supply voltage.